In a W-CDMA system, random access channels (RACHs) using Slotted ALOHA are present (see, for example, Non-Patent Document 1). A RACH is a channel for transmitting not wireless resources specific to and allocated to each mobile station but common wireless resources (a frequency band, a scrambling code, and time) shared among mobile stations in one cell. The RACH is a channel used to transmit signals that are relatively small in size and that are not transmitted continuously such as a control signal for notifying of a periodic measurement result or a control signal for requesting start of a data communication.
The RACH is constituted by two parts called a “preamble part” and a “message part”, and transmitted using orthogonal bit sequences called “signatures” so that a plurality of mobile stations can simultaneously access the RACH. 16 types of signatures are prepared, and each of the mobile stations selects one from among these signatures at random and uses the selected signature for scrambling the preamble and selecting a spreading code of the message part. Accordingly, if the mobile stations accidentally select the same signature and start random accesses at the same timing, collision of the message parts occurs. However, if mobile stations select different signatures, message parts can be received. In the latter case, however, a desired signal for one of the mobile stations becomes an interference signal for the other mobile station. Therefore, if the mobile stations transmit signals at the same transmission power, a so-called near-far problem occurs. Namely, a mobile station located farther from the base station, that is, a mobile station having a greater propagation loss suffers a higher interference from the other mobile station and a power for a desired wave attenuates, resulting in a greater deterioration in a signal to interference ratio (SIR).
Considering the near-far problem, as shown in FIG. 1, an open loop transmission power control is performed using preamble parts so that transmission power is set to as small power as possible in a range in which an SIR of the message part from each mobile station satisfies a desired value at the base station. Specifically, the open loop transmission power control has the following procedures.
One mobile station transmits a preamble at a predetermined initial power value Pinit [dBm]. At this time, a value calculated by the following equation is set to the initial power value Pinit [dBm] (see, for example, Non-Patent Document 2).Pinit=P_CPICH_Tx−CPICH_RSCP+UL_Interference+Constant_Value[dBm].
In the equation, P_CPICH_Tx [dBm] is a transmission power of a common pilot signal (CPICH: Common Pilot Channel) transmitted from the base station. UL_Interference and Constant_Value [dB] are predetermined power offsets and notified to each mobile station in a cell by a broadcast channel or the like as system parameters common to the mobile stations in the cell. Further, CPICH_RSCP [dBm] is a reception power level of the CPICH measured by each mobile station in a predetermined cycle.
As can be seen, the Pinit is decided according to the CPICH_RSCH, thereby eliminating the influence of the difference in propagation loss as much as possible and setting a reception level constant at the base station among the mobile stations.
Generally, however, a radio wave is susceptible to fading fluctuation generated by not only distance attenuation and shadowing but also movement of the mobile station in multipath environment. The fading fluctuation varies according to a carrier frequency. Due to this, in a W-CDMA FDD system using different frequency bands between an uplink and a downlink, a propagation loss measured in a downlink CPICH does not always coincide with that measured in an uplink CPICH. Moreover, because of presence of a measurement delay in the CPICH_RSCH, the propagation loss during transmission of a preamble greatly differs from that during measurement of the CPICH_RSCP depending on the movement of the mobile station, fading-caused drop or the like. Furthermore, the predetermined constants UL_Interference and Constant_Value are often set lower than optimum levels so as to suppress uplink interference. Due to such factors, a preamble reception power is insufficient and the base station is often incapable of detecting the preamble.
If the base station can receive the preamble, the base station transmits an acquisition indicator signal related to the preamble by a downlink common control channel after passage of a predetermined time ΔTack from a preamble transmission timing. At this time, if the base station permits the mobile station transmitting the preamble to transmit a message part, the base station transmits ACK to the base station. If the base station does not permit the mobile station to transmit the message part for such reasons as excess of the number of mobile stations from which the base station receives message parts, the base station transmits NACK to the mobile station.
On the other hand, the mobile station receives the downlink common control channel after passage of the predetermined time ΔTack from the preamble transmission timing and receives the acquisition indicator signal indicating ACK, the mobile station transmits the message part to the base station at a predetermined message part transmission timing. If the mobile station receives the acquisition indicator signal indicating NACK, then the mobile station notifies a higher layer of reception of the NACK and finishes the random access.
Furthermore, if the mobile station cannot receive the acquisition indicator signal at the predetermined timing, this means that the base station cannot receive the preamble. Therefore, the mobile station retransmits the preamble to the base station after a predetermined time. At this time, the mobile station retransmits the preamble at a preamble transmission power Ppre+tx(k+1) [dBm] that is a previous transmission power Ppre—tk(k) plus a preamble power increment step ΔPp [dB], i.e., performs so-called Ramp-up, where k indicates the number of times of retransmission of the preamble (k is set to 0 (k=0) at initial transmission).
The mobile station repeats the above-stated operations until receiving the acquisition indicator signal or the number of times of retransmission reaches a maximum number of times of retransmission K designated as a system parameter.
Likewise, for an EUTRA (Evolved Universal Terrestrial Radio Access) system currently hotly debated in 3GPP, it is considered to introduce uplink random access channels (see, for example, Non-Patent Document 3).
In relation to the EUTRA system, a wireless access method based on FDMA (Frequency Division Multiple Access) has been mainly discussed and random access on the premise that only one mobile station transmits signals in one frequency band and the like are considered. In this case, differently from the case where a plurality of mobile stations are allowed to access one channel in the same frequency band, the near-far problem does not occur. Due to this, a fixed power value common to the mobile stations in one cell can be set to a transmission power of each mobile station. In this case, however, it is necessary to set the transmission power so that the channel from even a mobile station located at a cell end has a sufficiently high quality at the base station. In other words, the mobile stations located at places other than the cell end transmit signals at excessive transmission power. Such a state unfavorably and unnecessarily increases interference with the adjacent cells if two adjacent cells use the same frequency band. Moreover, this unfavorably and unnecessarily increases power consumption of the mobile stations. Therefore, in the EUTRA, similarly to the WCDMA, it is preferable to make power setting based on the CPICH reception measurement value so that a mobile station having a higher propagation loss has a higher transmission power. However, the EUTRA has a smaller demerit of causing each mobile station to transmit a signal at excessive power than the WCDMA by as much as absence of the near-far problem. Due to this, it is proposed to set the transmission power so as to be able to satisfy a desired quality from initial transmission and to reduce a transmission delay in the RACH without performing the so-called power Ramp-up of starting an initial power lower than the power that can satisfy the desired quality and of gradually increasing the power as done in the WCDMA.
[Non-Patent Document 1] 3GPP TS25.214 v6.6.0 (2005-06) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 6)
[Non-Patent Document 2] 3GPP TS25.331 v6.6.0 (2005-06) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Radio Resource Control (RRC); Protocol Specification (Release 6)
[Non-Patent Document 3] 3GPP TS25.814 v0.2.0 (2005-08) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Layer Aspects for Evolved UTRA (Release 7)